When energy is locally deposited within a liquid, for example with an intense focused electromagnetic radiation (e.g., laser light) or with an electrical discharge through a spark, locally induced boiling of the liquid leads to a creation of a cavitation bubble that rapidly expands due to the high pressure within the vapor. When the bubble reaches its maximum volume where the internal pressure is lower than in the surrounding liquid the bubble starts to collapse. When the collapsing bubble reaches a given size it may rebound and the process repeats until there is insufficient energy for the bubble to rebound again. These violent cavitation oscillations lead to rapid streaming of liquid molecules around the cavitation bubble. It is also known that a cavitation bubble collapsing near a boundary forms a liquid jet directed at the boundary. Even more importantly, under appropriate conditions, an intense shock wave may be emitted during the bubble's collapse.
The strong mechanical forces associated with rapid bubble oscillations can break particles or remove particles from the surface, thus locally cleaning it. This effect is of interest for industrial applications, and as well in medicine. Laser-induced cavitation bubbles have been used in ophthalmology, cardiology, urology and dentistry. For example, laser pulses produce plasma with subsequent bubble formation for ocular surgery by photo-disruption. Laser induced lithotripsy fragments kidney stones through cavitation erosion. Laser pulses have been used to remove thrombus in obstructed arteries. In endodontics, laser activated irrigation is used to debride dental root canals. Laser induced cavitation may be also used for cleaning, debriding and disinfection of periodontal pockets, holes created during bone surgery, or surfaces of inserted implants.
In what follows, the terms “liquid” and “fluid” will be used interchangeably; furthermore, the term “cleaning” will be used to describe all or any of the potential mechanical, disinfecting or chemical effects of cavitation oscillations on surrounding environment (e.g., fragmentation, debridement, material removal, irrigation, cleaning, disinfection).
The principle lying behind cavitation phenomena is the difference in compressibility between a gas and a liquid. The volume of liquid hardly changes in response to a variation in pressure, whereas the volume of the gaseous interior of a bubble can change dramatically. Any contraction or expansion of the bubble is inevitably accompanied by a displacement of an equal volume of the much denser surrounding liquid. As a result, strong bubble's response in combination with the compressible interior can provide not only localized fluid motion but also tremendous focusing of the liquid kinetic energy. Of particular interest for cleaning are the shock waves which may form during the bubble's collapse. These shock waves spread through the volume at supersonic speeds, and interact disruptively with the surrounding environment (e.g., cavity walls). These waves are not only very effective in removing any contamination from the cavity surfaces but can also kill bacteria, leading to a partial or complete disinfection of the treated cavity.
In an infinite liquid, a secondary shock wave is emitted during the accelerated contraction of the bubble cavity. This secondary shock wave is to be distinguished from the primary shock wave which is sometimes emitted during the initial bubble expansion phase when laser energy is locally deposited into a liquid within a very short time of nanoseconds or less. In what follows, the term “shock wave” will represent the secondary shock wave emission only.
The (secondary) shock wave emission occurs as follows. At the initial moment of the bubble's contraction, the pressure inside the bubble equals that of the saturated vapor which is much less than the liquid pressure. Because of this transition, the bubble starts to contract and the bubble vapor pressure starts to grow. Initially, the bubble contraction is relatively slow. However, as the pressure rises, this leads to a vapor mass loss due to the condensation process on the bubble surface, accelerating the implosion even further. This ever faster acceleration results in a violent collapse of the bubble, leading to heating up of the vapor and, most importantly, to emission of a supersonic shock wave emanating from the collapsed bubble. And finally, when the vapor temperature reaches its critical value the condensation process stops, which leads to an even faster rise of the vapor pressure until the contraction stops and the bubble begins to rebound.
Whether the shock wave is emitted and with what amplitude depends among other parameters on the properties of the liquid and on the dimensions of the reservoir that contains the liquid. For example, it is known that for liquids with higher viscosity, bubble's oscillations are slower and last longer. In viscous fluids, the dynamics of the collapse is slowed down, reducing the energy of the shock wave. In highly viscous fluids, shock waves are not observed at all.
Similar dependence applies also with regard to the dimensions of the reservoir. In a free liquid, bubble oscillations can be accommodated by displacing the liquid at long distances. However, in a confined environment, a free expansion of the bubble is not possible, and the expansion and contraction of the bubble is slowed down by the added resistance to flow due to the impermeability and the no-slip condition on the reservoir's surface. This process delays the dynamics of bubble's expansion and implosion compared to a free liquid situation, and the period of the bubble's oscillations can be extended up to ten times. More importantly, because of the slowed down dynamics of the bubble's collapse, shock waves are weaker or do not occur at all. The influence of the reservoir's boundaries on the bubble's dynamics and shock wave formation can be roughly evaluated by introducing a containment factor (γ) representing a ratio, γ=dr/db, between the smallest dimension of the reservoir (dr) and the maximal dimension of the bubble (db) in the direction where the reservoir's dimension is smallest. Experiments have shown that the “containment” starts to exert significant influence on the bubble dynamics at containment ratios γ<3. Typically, at containment ratios γ<2, shock waves are not observed.
In biomedical applications, bubbles generated by a focused laser light have a typical maximum dimension between about 0.1 and 10 mm. For the purposes of understanding our invention it is important to note that this means that for most typically applied laser parameters and anatomic liquid reservoirs, such as blood vessels, ureter canals, or root canals, the containment factor is such that shock waves are weak or are not emitted at all. The cleaning effect of cavity oscillations is therefore limited to rapid liquid streaming and liquid jets, while the potential of much more violent shock waves is not utilized.
In particular, when performing, for example, dental endodontic treatments, removing debris from root canal surfaces and eliminating infection consists of adding various chemical solvents into a root canal, and then using a prior art laser irrigation method primarily to enhance the spreading of the chemical irrigant into hard to reach root canal areas. The use of potentially toxic irrigants is not desirable, however due to the absence of shock waves when performing prior art methods, using only water as the irrigating liquid is not sufficiently effective for cleaning and disinfecting root canals.
Accordingly, improved methods, techniques and technologies that can improve the cleaning efficacy of cavitation oscillations in small liquid reservoirs are desirable. The liquid reservoir may be a cavity, canal, vessel (e.g., a blood vessel), passage, opening surface which is to be cleaned or disinfected. In what follows the terms reservoir and/or cavity will be used interchangeably to describe any or all of the applicable liquid reservoirs.
Furthermore, with prior methods for cavitation cleaning it remains desirable to provide improved, more effective cleaning devices and/or methods wherein the electromagnetic radiation treatment parameters are adjusted and/or optimized to obtain strong secondary shock waves during bubble cavitation oscillations even in confined geometries (e.g., root canal systems, blood vessels, urinary tracts, periodontal pockets, surgical holes and the like). The material to be removed may include bacteria or debris (e.g., plaque, calculus, dirt, particulate matter, adhesives, biological matter, residue from a cleaning process, dust, stains) located on surfaces of the liquid reservoir, however, in other examples, the bacteria or debris may be suspended within the liquid filling the treated cavity.
The object of the present invention is to provide an improved cleaning system with a better conversion of electromagnetic energy into shock waves for improved cleaning results.
This object is solved by the cleaning system according to claim 1.
A further object of the present invention is to provide a method for operating said cleaning system to achieve improved cleaning results.
This object is solved by the method according to claim 24.
In what follows the terms electromagnetic radiation, light, laser or laser light will be used to describe any source of electromagnetic radiation or any electromagnetic radiation, where the source of the electromagnetic radiation may be a laser, laser diode, diode, lamp or any other source configured to produce the electromagnetic radiation having the wavelength that is substantially absorbed in the liquid, either in a linear or non-linear regime. A substantial or significant absorption means in the context of the present invention any absorption of the electromagnetic radiation energy to such an extent, that bubbles as described below are generated within the liquid. Said substantial or significant absorption covers in particular the interaction of laser light having a wavelength in a range from above 0.4 μm to 11.0 μm inclusive, including both wavelength in the range from about 1.3 μm to about 11.0 μm being highly absorbed in OH containing liquids, and wavelength in the range from about 0.4 μm to about 1.3 μm being weakly absorbed in OH containing liquids. However, any other suitable radiation and wavelength is covered like IPL (Intense Pulse Light) from flashlamp sources, in particular with wavelength above 1.3 microns or in the UV region when focused, as well as green flashlamp or diode light in blood. A further option within the invention is the use of a radiofrequency (RF) radiation source and its RF radiation. Within the scope of the present invention further wavelengths may be contemplated in particular in combination with liquids having added absorption enhancing additives.
For the purposes of describing present invention, the conditions under which a laser light is highly absorbed in a liquid is roughly divided into a linear, or thermal regime, and a non-linear regime. A linear absorption regime applies when laser pulse power density in a liquid is not high enough to result in the ionization or in other non-linear interactions with liquid molecules. Typically, lasers with pulse durations in a microsecond or millisecond range (from one microsecond to about 5000 μs), such as flash-lamp pumped free-generation Er:YAG lasers, operate in a linear regime. In this regime, the intensity I of a laser light exponentially diminishes with distance x within a liquid according to I exp (−kx), where k (in cm−1) is a linear absorption coefficient of the liquid at the particular laser wavelength. The absorption coefficient k and the corresponding penetration depth, 1=1/k, are strongly wavelength dependent. For example, the penetration depth of the Er:YAG laser wavelength of 2.94 μm in water is approximately 10−4 cm while the penetration depth of the Nd:YAG laser wavelength of 1.064 μm is 1 cm. According to this definition, laser wavelengths with 1>1000 μm in the linear regime are defined as “weakly absorbed” wavelengths. For water, and other OH-containing liquids, the applicable range of highly absorbed wavelengths extends from about 1.3 μm, inclusive, to about 11 μm, and the applicable range of weakly absorbed wavelengths extends from about 0.4 μm to 1.3 μm. In another example, when the liquid is blood, the 532-nm wavelength of a frequency doubled Nd:YAG laser, the 585 nm wavelength of the pulsed-dye laser or the 568 nm wavelength of the Krypton laser, are of interest since they are strongly absorbed in blood's oxyhemoglobin, with their k being approximately within 300-500 cm−1 range.
At extremely high laser power densities, on the order of about of 1010-1011 W/cm2, an “optical breakdown” as a result of the ionization of liquid molecules may occur, leading to an abrupt increase in liquid's absorption. In this, non-linear regime, a high absorption of laser light is observed even for weakly absorbed wavelengths, i.e., for wavelengths which have a long penetration depth p in the linear regime. Non-linear conditions are typically achieved with high pulse power Q-switched laser beams, with pulse durations (tp) in a nanosecond range (from one nanosecond to about 100 ns), especially when these beams are focused into a sufficiently small volume of the liquid. But other high pulse power lasers with even shorter pulse durations, in the picosecond and femtosecond range, have been used to generate cavitation in liquids as well. It is to be appreciated that when an optical path of a weakly absorbed high pulse power laser beam has a focal point located within a liquid, the beam will propagate within the liquid without being appreciably absorbed until it reaches the focal region where the laser power density becomes sufficiently high for non-linear effects to occur. It is only at this point that a bubble formation will occur.
When a pulsed laser beam which is highly absorbed in liquids, either in a linear or non-linear regime, is delivered to such a liquid a bubble generation occurs. For laser pulse durations longer than approximately 500 nanoseconds there are no primary shock waves created in the liquid during the bubble expansion. Instead, the energy stored in the bubble is converted into acoustic energy only after the bubble reaches its maximum size (Amax1), and the difference in pressures forces the bubble to collapse. Therefore, lasers operating in a linear regime are most suitable for performing present invention since more energy for secondary shock wave emission has remained available in the bubble before it starts to collapse. However, present invention can be of benefit also for applications where cavitation is generated with lasers operating in a non-linear regime.
In summary, when a pulsed laser beam which is highly absorbed in a liquid, either in a linear or non-linear regime, is delivered to such a liquid, a bubble oscillation sequence develops with a very short temporal oscillation period (TB) in the range from about 100 μsec to about 1000 μsec. The oscillation is damped and lasts for only a few rebounds due to the bubble's energy being spent for heating, moving and displacing the liquid, and under appropriate conditions, also for emitting shock wave acoustic transients. For the purposes of cleaning it is desirable that as much as possible of the bubble's energy is spent in the emission of violent shock waves during the contraction phases of the bubble's oscillation, and preferably at least during the first bubble's contraction phase when the bubble's energy is still high. However, in highly viscous liquids and/or when the reservoir-bubble containment ratios (γ) are small, more energy is wasted for overcoming viscous damping and/or to fight against the resistance of the water which has to be displaced in the small reservoir. Consequently, the bubble's contraction is slowed down, resulting in a lower amplitude shock wave or no shock wave at all.
A laser system can be configured to deliver the laser light to a liquid in a contact or a non-contact manner. In a contact scenario, the laser light is delivered to the liquid through an exit surface of an optical exit component (e.g., fiber, fiber tip, optical window, lens) which is at least partially submersed into the liquid. The laser light's focus is located at the exit surface of the exit component, and the bubble develops in a contact with the exit surface of the submersed optical exit component.
In a non-contact scenario, the optical exit component is configured to be positioned above the surface of the liquid reservoir, with the laser energy being directed through air and possibly other transparent materials (such as, for example an eye lens in case of ophthalmic applications) into the liquid reservoir. In a non-contact scenario, the beam is substantially focused to a point located bellow the liquid surface by means of an appropriate focusing device, and the resulting bubble does not develop in a contact with the optical exit component.
It is to be appreciated that the contact manner is more suitable for configurations when laser light is absorbed in a linear regime, and the non-contact manner is more suitable for configurations when laser light is absorbed in a non-linear regime. However, either of the delivery manners can be used in a linear or a non-linear regime.
Present invention is based on our discovery that when the energy is delivered to a liquid in a set of a minimum of two individual laser pulses (a prior and a subsequent pulse), follow temporally each other by an appropriate pulse repetition time (Tp), the pulse repetition time TP being the time period from the beginning of one single pulse p to the beginning of the next, subsequent pulse p, a shock wave is emitted by the prior bubble, i.e., the bubble resulting from the prior laser pulse, even in situations when no shock wave is emitted by the bubble when only one laser pulse is delivered to the liquid. This observation is explained by the fact that the liquid pressure exerted on the prior bubble by the expanding subsequent bubble, i.e., the bubble resulting from the subsequent laser pulse, forces the prior bubble to collapse faster, which leads to the emission of a shock wave by the prior bubble.
There are two conditions that need to be fulfilled in order for the above described effect to be observed. The first condition requires that the subsequent bubble starts to develop when the prior bubble is already in its implosion phase, with its size having been reduced from its maximum size (Vmax1) to a size in a range from about 0.7×Vmax1 to about 0.1×Vmax1. And secondly, the laser energy of the subsequent pulse must be delivered at a location nearby the prior bubble but not within the prior bubble. In the opposite case, the laser beam of the subsequent pulse will not be initially absorbed in the liquid but shall first pass through the prior vapor bubble and will be absorbed at the prior bubble's wall area generally opposite to the direction of the laser beam. This would result in extending the length of the prior bubble in the direction of laser beam emission, and would therefore shift the bubble's dynamics from the contraction to expansion phase, effectively preventing the formation of a shock wave.
When both laser pulses are focused to the same spot within the liquid, the second condition can be fulfilled only when the subsequent laser pulse is emitted when the prior bubble has already moved sufficiently away from its initial position, i.e., from the point in the liquid where laser energy is being locally absorbed within the liquid. Such movement occurs naturally in contact delivery scenarios where during its contraction phase the bubble separates and moves away from the exit surface of the optical exit component. In one of the embodiments, a highly absorbed wavelength may be delivered into a narrow, tube like reservoir, such as a root canal or a blood vessel, by a submerged fiber or fiber tip. In this configuration, the fluid dynamics has been observed to be such that during its contraction phase the bubble separates from the fiber end and moves away from the fiber. This allows the subsequent bubble to develop at the fiber end separately from the prior bubble, and by its expansion to cause the surrounding liquid to exert pressure on the prior bubble during its contraction.
The bubble may move away from the laser's focal point also in non-contact scenarios, providing that the confined reservoir wall's geometry is asymmetrical with regard to the bubble, and the resulting asymmetrical liquid flow shifts the bubble away from its original expansion position.
In another embodiment, the second condition may be fulfilled by physically moving the fiber to a different position within the liquid during the repetition time of the two laser pulses. In yet another embodiment, it is the laser focal point which may be moved in between the pulses, for example by a scanner.
It is to be appreciated that the invention is not limited to the emission of only two subsequent pulses within a pulse set. A third pulse following a second laser pulse, and fulfilling both conditions, may be delivered resulting in an emission of a shock wave by the previous (second) bubble. Similarly, an nth subsequent laser pulse will result in an emission of a shock wave by the (n−1)th bubble, and so on as further laser pulses are being added to the set of pulses. The more laser pulses are delivered in one pulse set, the higher is the laser-to-shock wave energy conversion, with the energy conversion efficiency being proportional to the ratio (n−1)/n where n is the total number of laser pulses delivered in a pulse set. Additionally, repetitive cavitations and shock wave emissions generate an ever increasing number of longer persisting gas (e.g., air) micro-bubbles within a liquid. These micro-bubbles compress and expand under the influence of cavitation oscillations and shock waves, and thus improve the overall cleaning efficacy by contributing to the high-speed fluid motion.
Our experiments show that the separation between the two laser pulses (Tp) should not deviate substantially from the optimal separation (Tp-opt) in order for the shock wave to occur. The optimal repetition time (Tp-opt) is the pulse repetition time where the subsequent bubble starts to develop when the prior bubble has already contracted to a size in a range from about 0.7×Vmax1 to about 0.1×Vmax1, preferably in a range from about 0.5×Vmax1 to about 0.1×Vmax1, and expediently in a range from about 0.5×Vmax1 to about 0.2×Vmax1. When the same laser device is intended to be used for cleaning differently sized cavities, containing different liquids, and with different laser parameters, this poses a challenge since the bubble oscillation time (TB), and consequently the required pulse repetition time (Tp-opt) depends critically on these conditions, being longer for higher laser pulse energies and pulse durations, more viscous liquids and smaller reservoirs.
In one of the embodiments of our invention, the laser system comprises a feedback system to determine a bubble oscillation dimension or amplitude of the prior vapor bubble generated within the liquid. Furthermore, the laser system comprises adjusting means for adjusting the pulse repetition time Tp to achieve at least approximately that the subsequent bubble, i.e., the bubble generated by the subsequent laser pulse, starts to expand when the prior bubble has already contracted to a size in a range from about 0.7×Vmax1 to about 0.1×Vmax1, preferably in a range from about 0.5×Vmax1 to about 0.1×Vmax1, and expediently in a range from about 0.5×Vmax1 to about 0.2×Vmax1. The feedback system preferably comprises an acoustical, a pressure, or an optical measurement sensor for sensing the bubble size A. As a result of the bubble oscillation sensing, the laser pulse repetition time Tp might be manually adjusted by the user to be approximately equal to Tpopt. However, in a preferred embodiment, the feedback system and the adjusting means are connected to form a closed control loop for automatically delivering a subsequent laser pulse at the moment when the feedback system has detected that the size of the prior bubble has contracted to a size in a range from about 0.7×Vmax1 to about 0.1×Vmax1, preferably in a range from about 0.5×Vmax1 to about 0.1×Vmax1, and expediently in a range from about 0.5×Vmax1 to about 0.2×Vmax1.
In yet other embodiments of the present invention, the laser system is configured to deliver laser energy in innovative “SWEEP” pulse sets wherein the pulse repetition time Tp is “swept” within each pulse set or from pulse set to pulse set across a sufficiently large range of pulse repetition time values (Tp) that the optimal pulse repetition time (Tp-opt) is reached at least once during each sweep. In an alternative SWEEP mode the electromagnetic radiation system and/or its operating method are adjusted to generate and deliver multiple pairs of two pulses, and wherein from pair of pulses to pair of pulses the pulse energy of each second pulse is varied in a sweeping manner.
More generally, various shortcomings of prior medical devices and methods (for example, endodontic treatments) can be addressed by utilizing a medical and dental treatment system or other exemplary system configured in accordance with principles of the present disclosure. Outside of the medical and dental fields, control of bacteria or other undesirable matter, such as dirt, particulate matter, adhesives, biological matter, residues, dust and stains, in various systems is also important. Further, cleaning and removal of various materials from surfaces and openings may be required for aesthetic or restoration reasons.
For following the above mentioned inventive findings, the individual pulses as they are known in the prior art are in a preferred embodiment replaced by inventive pulse sets. The individual pulses are combined to pulse sets consisting of a minimum of two and maximally 20 individual pulses, with the intra-set pulse repetition times in the range from 50 μsec to 900 μsec, and the pulse sets being temporally separated from each other by at least 10 ms.
The proposed laser system and method may be applied to any kind of human or animal cavity, or even industrial cavity.